'Cornish pasty-shaped'

Its normal habitats are the warm seas off Florida Keys, the Gulf of Mexico, Indian Ocean, Caribbean and Pacific.

Dr Peter Richardson, MCS Biodiversity Programme Manager, said: "Between 2003 and 2006 the MCS jellyfish survey received less that 10 reports of Portuguese men-of-war, but since then sightings have increased, to over 60 reports in 2009."

Portuguese men-of-war are about 30cm long and 13cm wide.

The MCS describes the men-of-war as a "Cornish pasty-shaped, transparent purple float with blue, tentacle-like 'fishing polyps' that hang below the float can be tens of meters in length".

Dr Richardson said they "deliver an agonising and potentially lethal sting".

"Because a stranded Portuguese man-of-war looks a bit like a deflating purple balloon with blue ribbons attached, it may attract the curiosity of children."

The sightings come just days after swimmers and surfers were warned that Portuguese men-of-war had been spotted on the Irish coast.